1. Field of the Invention
The invention relates to a semiconductor device including a bonding pad, and also relates to a wire bonding method.
2. Description of the Related Art
A wire bonding method has been available as a method of electrically connecting a semiconductor device to an external circuit. In the wire bonding method, a wire whose one end is bonded to the external circuit is extended to above a bonding pad formed on a surface of the semiconductor device, and bonded on the bonding pad. After the wire is bonded on the bonding pad, the bonded wire is cut. The bonding pad will be hereinafter simply referred to as a “pad”.
There is a semiconductor device that requires various pads. For example, a semiconductor device for electric power control requires at least one electric power pad and a signal pad. A wire, through which the controlled electric power flows, is bonded on the at least one electric power pad. A wire, which transmits a signal to switch between on and off of the semiconductor device, is bonded on the signal pad. In addition to the signal pad as described above, the semiconductor device may include a pad on which a wire that transmits a signal corresponding to an amount of current flowing through the semiconductor device is bonded, or a pad on which a wire that transmits a signal corresponding to a temperature of the semiconductor device is bonded. For example, Japanese Patent Application Publications No. 2005-129826 (JP-A-2005-129826) and No. 7-326711 (JP-A-7-326711) describe a semiconductor device that includes a pad on its surface, and a device that bonds a wire to the pad.
It is difficult to form a semiconductor structure required for operating the semiconductor device in a portion of a semiconductor substrate below the signal pad on which the wire that transmits the signal is bonded. This is because i) unstable operation of the semiconductor device due to the voltage applied to the signal pad needs be avoided; and ii) a thick oxide layer needs to be formed to insulate the semiconductor structure from the signal pad in order to detect a temperature.
In a high-voltage semiconductor device, an electric field tends to concentrate in a terminal region of the semiconductor substrate, and this causes deterioration of voltage-resistant performance of the semiconductor device. In order to avoid this, there has been proposed the technology in which a field limiting ring (FLR) is disposed on an outer peripheral portion of a surface of the semiconductor substrate to continuously extend along an outer periphery of the semiconductor substrate. In order to improve the voltage-resistant performance of the semiconductor device using the FLR, it is necessary to dispose the semiconductor structure, which is required for operating the semiconductor device, in a region inside the FLR.
In the semiconductor device in which the FLR is provided so as to improve the voltage-resistant performance, the signal pad needs to be provided in the region inside the FLR. If a conductive region, which is provided to secure electrical connection between the signal pad and a semiconductor region constituting the semiconductor structure, is disposed across the FLR, it is difficult to improve the voltage-resistant performance using the FLR.
The following is the summary of what is described above. (1) When the semiconductor device includes the FLR that is disposed on the outer peripheral portion of the surface of the semiconductor substrate to continuously extend along the outer periphery of the semiconductor substrate in order to improve the voltage-resistant performance, the semiconductor structure needs to be disposed in the region inside the FLR. Therefore, it is not possible to use the entire area of the semiconductor substrate solely for forming the semiconductor structure. (2) In addition to the semiconductor structure, the signal pad needs to be disposed in the region inside the FLR. However, it is not possible to provide the semiconductor structure below the signal pad. Therefore, because the signal pad needs to be disposed in the region inside the FLR as well as the semiconductor structure, the effective area of the semiconductor substrate for forming the semiconductor structure is further reduced. In the semiconductor device according to the related art, the effective area for forming the semiconductor structure required for operating the semiconductor device is reduced.
If the process involves only wire bonding, it is possible to bond the wire on a small-sized pad. However, if the process involves cutting of the wire bonded on the pad as well, the pad needs to be large enough. When the wire is to be cut, the wire is pressed against the pad, using a tool, at the position where the wire is bonded on the pad, and then the wire is cut (it should be noted, although it is apparent, that the wire is cut at an end that is opposite to an end connected to the external circuit). When the wire is cut, the wire-cutting process leaves a drag mark on the surface of the semiconductor device. Therefore, it is necessary to prevent the surface of a protective layer, which is formed to cover the surface of the semiconductor device, from being damaged by the drag mark. In order to do so, it is necessary to use a pad that is large enough to cover a formation area in which the drag mark may be formed. The pad on which the wire is bonded and cut needs to be made larger than the pad on which the wire is bonded, but does not need to be cut.